Our present invention relates to a process for the removal of carbon dioxide, sulphur compounds, water and aromatic and higher aliphatic hydrocarbons from industrial gases. Industrial gases in this context are, in particular, natural gas and/or raw synthesis gas. Sulphur compounds according to the present invention are, in particular, hydrogen sulphide and organic sulphur compounds present in natural gas and raw synthesis gas.
Carbon dioxide and hydrogen sulphide are, among others, so-called sour gas components present in natural gas and raw synthesis gas. Apart from sour gas components, such as hydrogen sulphide and carbon dioxide, water vapor must also be removed from natural gas. If the natural gas also contains aromatic hydrocarbons and higher aliphatic hydrocarbons, these can be recovered as a useful substance by absorption using an absorbent. In such case, care must be taken when regenerating the absorbent to ensure that none or only minute quantities of these hydrocarbon components are released or emitted into the environment.
It is known, in principle, that hydrogen sulphide and carbon dioxide can be removed from industrial gases, such as natural gas and synthesis gas, with the aid of chemical absorbents, e.g. ethanolamines, alkaline salt solutions and the like. When using chemical scrubbing agents, the absorbent is present in the form of an aqueous solution. It is, therefore, necessary to dry the industrial gas downstream of the absorption step which removes hydrogen sulphide and carbon dioxide. This is effected preferably by glycol scrubbing or by molecular sieve adsorption. When glycol is used for drying, for instance, aromatic hydrocarbons and higher aliphatic hydrocarbons are partly removed from the natural gas in addition to water. In regenerating the scrubbing solution, these hydrocarbons are emitted disadvantageously to the environment. Statutory requirements allow only limited toleration of such emissions.
U.S. Pat. No. 3,773,896 describes the use of morpholine derivatives as absorbents for hydrogen sulphide and carbon dioxide. By employing thermal regeneration of the absorbent at a temperature of 80xc2x0 C., 70% to 80% of the absorbed sulphur compounds and 55% to 65% of the absorbed carbon dioxide can be removed from the absorbent. This limited desorption of the dissolved gases from the laden absorbent is disadvantageous, because the gas portions remaining in solution impede the absorption of carbon dioxide, sulphur compounds and water from the raw gas and prevent the achievement of low residual concentrations of these substances in the treated gas.
It is also known from DE 1 568 940 A1 that morpholine derivatives have a high affinity for aromatic hydrocarbons and can, therefore, be used for the removal of these aromatic hydrocarbons from natural gas.
In addition, it is known that physical absorbents, such as polyethylene glycol dimethyl ether, propylene carbonate or methanol, can be used for the removal of hydrogen sulphide and carbon dioxide from industrial gases. In order to remove the major portion of the dissolved sour gas components from the physical scrubbing fluids, the use of inert stripping gases, such as nitrogen, is also known for stripping the dissolved gas components. In these cases, the inert stripping gas flows counter-current to the absorbent laden with sour gas components. The dissolved gas components are entrained by the inert gas and withdrawn from the head of the desorption column.
It is further known that water can be added to the absorbent, the water being vaporized by indirect heating in the bottom of the desorption column. In this way, a quantity of stripping steam is generated as is required for expelling the dissolved gas components from the absorbent. This process is used especially when it is necessary to keep the evaporation temperatures in the column bottom low in order to prevent thermal decomposition of the absorbent. Because of the total miscibility of morpholine derivatives with water, this process can also be used for the removal of sour gas components from the morpholine derivatives. However, the last-mentioned process has a considerable disadvantage, in that the water portion in the absorbent does not contribute towards the sour gas absorption and has to be transported as ballast in the absorbent cycle. Moreover, due to the absorbent being pre-laden with water, an effective removal of the water portion from the industrial gas is not possible.
It is the object of the present invention to provide a process that permits, on the one hand, the complete removal of various substances, especially sour gas components, water and aromatic and higher aliphatic hydrocarbons with the aid of an adsorbent, and, on the other hand, the complete regeneration of the absorbent or, in other words, virtually total removal of the above-named components from the absorbent.
To meet this requirement , the present invention describes a process for the removal of carbon dioxide, sulphur compounds, water and hydrocarbons from industrial gases, in which
sour gas components, water and aromatic and higher aliphatic hydrocarbons are removed at an elevated operating pressure from the gas to be treated,
at least a morpholine derivative is used as the absorbent,
the absorbent laden with the absorbed components is regenerated with the aid of a stripping gas, and
the stripping gas is generated by partial evaporation of the laden absorbent.
It is an integral part of the invention that the absorbed components are partly removed from the absorbent by pressure relief of thermal regeneration. It is also an integral part of the invention that the stripping gas is generated by partial evaporation of the laden absorbent at negative pressure.
Within the scope of the invention, absorption at an elevated operating pressure means that the absorption takes place at a pressure above standard pressure or above 1 bar. In other words, an absorption column is used operating at a pressure above 1 bar. In a preferred embodiment of the invention, the absorption is performed at an operating pressure of 10 to 150 bar.
A preferred embodiment of the invention provides for the use of at least one morpholine derivative from the group of N-formylmorpholine, N-acetylmorpholine, and N-propionylmorpholine as the absorbent. It is within the scope of the invention that only N-formylmorpholine (NFM) or only N-acetylmorpholine (NAM) is used. A very preferred embodiment of the invention provides for the use of a mixture of two morpholine derivatives. According to a preferred embodiment, which has a special significance within the scope of the invention, a mixture of N-formylmorpholine (NFM) and N-acetylmorpholine (NAM) is used as the absorbent. The mixing ratio can be in the range of 10 to 90 parts (mass) of NFM to 90 to 10 parts (mass) NAM, referred to 100 parts (mass). A preferred embodiment of the invention provides for the use of 30 to 70 parts (mass) NFM and 70 to 30 parts (mass) NAM, such that the total is 100 parts (mass). The absorbent can also contain 0.1 to 5 parts (mass) of water.
A very preferred embodiment of the invention provides for at least one absorber and an absorbent temperature of xe2x88x9220xc2x0 C. to +40xc2x0 C. One embodiment of the invention provides for at least one absorber and an absorbent temperature of xe2x88x9215xc2x0 C. to +10xc2x0 C. Another preferred embodiment of the invention provides for an absorbent temperature of xe2x88x9220xc2x0 C. to 0xc2x0 C.
According to a very preferred embodiment of the invention, water and hydrocarbons are absorbed in a separate absorption stage. Such a separate absorption stage would appropriately be an upstream absorption column. It would, however, also be within the scope of the invention for the separate absorption stage to be the lower part of an absorption column, the sour gas components being absorbed in the upper part of the column. The preferred operating pressure of the separate absorption stage for the absorption of water and hydrocarbons is 10 to 150 bar and the absorbent is appropriately a mixture of N-formylmorpholine (NFM) and N-acetylmorpholine (NAM). The preferred mixing ratio of NFM and NAM is the advantageous ratio mentioned above. It is within the scope of the invention that the temperature of absorbent in the separate absorption stage for the absorption of water and hydrocarbons is xe2x88x9220xc2x0 C. to +40xc2x0 C. One embodiment of the invention provides for an absorbent temperature of xe2x88x9215xc2x0 C. to +10xc2x0 C. The preferred temperature is xe2x88x9220xc2x0 C. to 0xc2x0 C.
According to a preferred embodiment of the invention, the sour gas components are absorbed in a separate absorption stage. This separate absorption stage for the absorption of sour gas components is preferably an absorption column arranged downstream of the separate absorption stage for the absorption of water and hydrocarbons. However, within the scope of the invention the separate absorption stage for the absorption of sour gas components can be the upper part of an absorption column, the lower part being used for the absorption of water and hydrocarbons. It is expedient that the separate absorption stage for the absorption of sour gas components or the respective absorption column be operated at a pressure of 10 to 150 bar. The preferred absorbent is a mixture of N-formylmorpholine (NFM) and N-acetylmorpholine (NAM), the preferred mixing ratio being the ratio already described above. Within the scope of the invention, that the temperature of the absorbent used in the absorption stage for the absorption of sour gas components can be xe2x88x9220xc2x0 C. to +40xc2x0 C. An embodiment of the invention provides xe2x88x9215xc2x0 C. to +10xc2x0 C. for this temperature. A preferred embodiment provides for a temperature of xe2x88x9220xc2x0 C. to 0xc2x0 C.
It is within the scope of the invention that the components to be removed from the industrial gas, namely carbon dioxide, sulphur compounds, water, aromatic hydrocarbons and higher aliphatic hydrocarbons be absorbed by the morpholine derivatives as completely as possible. The absorbent laden with these absorbed components is regenerated with the aid of a stripping gas. To this end, the laden absorbent is fed to a desorber or desorption column completely or as a part stream, where the regeneration of the laden absorbent takes place in accordance with the invention. The stripping gas required for regenerating the absorbent is generated according to the invention by partial evaporation of the laden absorbent stream or part stream withdrawn from the absorption unit. in a preferred embodiment of the invention, the partial evaporation of the laden absorbent stream is performed at a pressure of 0.1 to 0.7 bar (absolute), preferably at a pressure of 0.2 to 0.4 bar (absolute). It is expedient that the partial evaporation is performed in the sump of a desorption column. Hence it is within the scope of the invention that the regeneration of the absorbent in the desorber or the desorption column is achieved by partial evaporation at a partial vacuum. Among other things, this avoids inadmissible high temperatures that would jeopardize the stability of the absorbent. The vaporized portion of the absorbent that forms during its partial evaporation serves as the stripping agent used for the virtually total removal of the absorbed components of the industrial gas to be treated, such as carbon dioxide, hydrogen sulphide, aromatic hydrocarbons and higher aliphatic hydrocarbons from the liquid portion of the absorbent.
According to a very preferred embodiment of the invention, the partial evaporation of the absorbent is performed in the sump of a desorption column at partial vacuum. Thus, with this embodiment of the invention, the stripping gas is generated in the sump of the desorption column or in the desorber bottom.
The invention is based on the surprising discovery that, upon performing the process according to the invention, a virtually total removal of sour gas components, water, as well as aromatic hydrocarbons and higher aliphatic hydrocarbons is possible without any problems. Moreover, upon performing the process, virtually total regeneration of the absorbent laden with the above-mentioned components is surprisingly possible. The sour gas components, water portions and hydrocarbons absorbent are surprisingly virtually totally removed from the absorbent.
A person versed in the art would not expect that such a complete regeneration of the absorbent laden with several components is possible using stripping gas generated according to the invention by partial evaporation of the absorbent.
By employing the partial evaporation of the absorbent according to the invention, preferably in the desorber sump under partial vacuum, the energy required for regeneration is considerably reduced as compared with that required for the known regeneration described earlier, in which the stripping gas is generated by evaporation of the water added to the absorbent. This results from the fact that the quantity of water added to a physically acting absorbent does not contribute towards the absorption of carbon dioxide and sulphur components, thus increasing the recirculated quantity of absorbent which, in turn, results in higher energy requirements or energy costs as compared to the process according to the invention. Compared with the known processes, the energy necessary for the evaporation of the stripping gas from the absorbent is considerably lower, because the heat of evaporation of the morpholine derivatives is only xc2xc of the heat of evaporation of water.
In a very preferred embodiment of the invention, the gas components recovered from the absorbent are withdrawn at the head of the desorber or desorption column, the water and hydrocarbons being condensed by partial condensation, an aqueous liquid phase and a hydrocarbon-bearing liquid phase being obtained. The hydrocarbon-bearing liquid phase is understood to mean a liquid phase that contains aromatic hydrocarbons and higher aliphatic hydrocarbons. It is thus within the scope of the invention that the partial condensation in a partial condenser yields two separate liquid phases. In a preferred embodiment of the invention, part of the aqueous liquid phase of the condensate is recycled to the head of the desorbers or desorption column for absorbent recovery. A part stream of the aqueous liquid phase, the quantity of which corresponds to the quantity of water absorbed from the gas stream to be treated, is preferably removed. In a preferred embodiment of the invention, the hydrocarbon-bearing liquid phase, i.e. the phase containing the aromatic and higher aliphatic hydrocarbons, obtained in partial condensation is removed as a product containing valuable substances.
In a very preferred embodiment of the invention, the sour gas components withdrawn from the desorber are compressed to the operating pressure of the absorption unit. The sour gas components leaving the partial condensation unit or the partial condenser are preferably recycled to the absorber. The sour gas components are, in particular, carbon dioxide, sulphur components and the residual light hydrocarbons. The sour gas components still contain minor quantities of aromatic and higher aliphatic hydrocarbons. As an expedience, the pressure of the entire sour gas stream leaving the partial condensation unit or partial condenser is raised to the pressure of the absorption unit with the aid of a compressor and the compressed sour gas stream is then recycled to the absorption unit. It is within the scope of the invention that this compressed sour gas stream is fed to the separate absorption stage for the absorption of hydrocarbons. In this way, the residual quantities of aromatic and higher aliphatic hydrocarbons are effectively removed. This embodiment has special advantages, as the process according to the invention completely avoids the detrimental emission of hydrocarbons. Alternatively, the sour gas stream leaving the desorber, and preferably the sour gas stream leaving the partial condenser, can be fed to the suction side of recycle gas compressor which is always present when a physical scrubbing system is used. In this way, the sour gas stream is fed, together with the recycle gas stream, by the recycle compressor to the absorption unit. If a portion of the sour gas components is withdrawn in the regeneration unit, especially if the sulphur components and carbon dioxide are to be won separately, it is possible to withdraw the sour gas fraction via a flash vessel arranged upstream of the desorber or the desorption column at any temperature and pressure. In the event that carbon dioxide and sulphur components are won separately, it is within the scope of the invention that the carbon dioxide can be withdrawn from the absorbent stream not being fed to the desorber in one or more thermal stages by thermal regeneration at any desired pressure.
The absorbent regenerated in accordance with the invention is preferably cooled downstream of the regeneration unit and subsequently returned to the regeneration unit. Surprisingly, it is possible with the process according to the invention to remove carbon dioxide, water, aromatic and higher aliphatic hydrocarbons almost completely from the gas to be treated. Hydrogen sulphide can be removed by the process to a residual content of 1 ppmv, carbon dioxide to a residual content of 10 ppmv and the water to a residual content of 1 ppmv.